A CRACKED nuclear reactor has led to more than half of the Royal Navy’s frontline attack submarines being taken out of service.

Experts warned the fault in the reactor of HMS Trenchant was so serious that the Trafalgar fleet may never sail again. The fracture is being treated as an “irreparable generic fault” that will prevent it from being able to carry out normal duties.

It means Britain may be forced to beg for international support in protecting our four Vanguard submarines, which carry Britain’s Trident nuclear deterrent.

Britain deploys seven SSN nuclear-powered hunter-killers. Last week it was revealed that the four older Trafalgar-class submarines, Trenchant, Torbay, Triumph and Talent, were out of action due to repairs and maintenance.

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A crack was discovered in the nuclear reactor of the submarine HMS Trenchant

However, sources have confirmed that the Trenchant was docked after engineers discovered a fracture at the heart of its nuclear reactor while at sea. The fracture is on a metal weld connecting a coolant pipe to the reactor pressure vessel. It is currently less than 100mm long, classing it as a critical fault, but if it grows it would be classified “catastrophic”. Because it sits within a tank of water to shield the radiation it is extremely difficult for engineers to get at.

A Navy source said yesterday: “The fracture has appeared in a pipe weld and our safety measures are very high. We will not sail until they are checked.”

The Defence Safety Nuclear Regulator will decide the fate of the vessels, but nuclear engineers warned the fault was likely to be “terminal” – and may also affect the other three vessels because they are so old.

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The seriousness of the fault could render the submarine inert

Nuclear engineer John Large, who in 2000 helped to repair a similar but less drastic fault on HMS Tireless, said: “If the fracture is in the reactor pressure valve, it would indicate that it’s on a weld. This would be very difficult to access.

A crack has developed. It is at a critical level and it has been detected

John Large, Nuclear engineer

“It sits in a tank of water, to shield the radiation. Over time the rector pressure vessel becomes increasingly radioactive, and most of these vessels are approaching the end of their lifespan. The probability is that the Trenchant is non-repairable and that would be a disaster. A problem like this is likely to develop in each boat, because they are of similar ages.”

Once such a fracture appears it is only a matter of time before restarting the reactor would make it grow to 100mm.

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Under the so-called break preclusion system, the reactor is designed never to require repair and never to fail. If it does fail it can’t be replaced.

Mr Large added: “A crack has developed. It is at a critical level and it has been detected. Like when you have a chip on a windscreen, it is difficult to reliably predict when it will suddenly grow and shatter.”

Sources suggested yesterday that the Royal Navy would be “pulling out all the stops” to deploy the boats once more.

However, deploying any submarine with such a fault will render it a “lame duck”, as it would be forced to avoid the strain caused by sudden shifts in temperature, or risk catastrophic failure.

Mr Large said: “These submarines are hunter killers – they are effectively sports cars required to manoeuvre rapidly. They need to run between ‘State A,’ idling, and ‘State B’, full power battle state, quickly if they are to avoid detection or even torpedo strikes. You cannot do this if there is a crack.”

The Trafalgar submarines have had their service extended by 10 years due to ongoing delays in the construction and delivery of the new Astute class. But of Britain’s three Astutes, two are undergoing trials while the third is being repaired after a collision in Gibraltar.

In 2013 a Ministry of Defence report revealed two Trafalgar class boats had been operating with a safety defect which put the vessels and crew at “serious” potential risk. Rear Admiral Chris Parry warned: “The SSNs are gatekeepers that detect and protect the UK from intrusive patrols by Russia and other opponents. They also provide vital protection for our Vanguard submarines.

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“Our deterrent is independent so, if our SSNs are not available, there are limits to what we can ask even the US to do because our Vanguard submarines need to maintain the secrecy of their operations. It does not help that the Government scrapped our Nimrod Maritime Patrol aircraft.

“Instead of gatekeepers I suspect that we would have to ask the US to act as linebackers to keep any hostile snooping submarines at a distance.”

An MoD spokesman denied that the submarines would be permanently out of action. He said: “It is untrue to suggest that HMS Trenchant or the rest of the T-class subs are unable to deploy again.”